Physical Downlink Control Channel (PDCCH) is used for transmission of scheduling indicator, resource allocation indicator, transmission mode indicator and other L1/L2 control information in Long Term Evolution (LTE). In the earlier releases (Releases 8/9/10 or Rel 8/9/10) of LTE, PDCCH is multiplexed with Physical Downlink Shared Channel (PDSCH) through Time Division Multiplexing (TDM) so that PDCCH occupies first 1-4 Orthogonal Frequency Division Multiplexing (OFDM) symbol(s) of each downlink sub-frame. The particular number of occupied symbols depends upon the amount of control information carried over PDCCH, bandwidth of the system and type of the sub-frame.
In LTE Rel8/9/10, PDCCH is transmitted based upon a Cell-Specific Reference Signal (CRS). When the number of CRS ports is 1, PDCCH is transmitted via a single port, port 0; when the number of CRS ports is 2, PDCCH is transmitted through Space-Frequency Block Coding (SFBC); and when the number of CRS ports is 4, PDCCH is transmitted through the combination of Space-Frequency Block Coding (SFBC) and Frequency Switched Transmit Diversity (FSTD).
Along with further evolvement of transmission technology and expansion of network deployment, there is a new requirement by the LTE system for transmission of downlink control information, and the original PDCCH mechanism also has conic to suffer from numerous problems as follows.
1) Capacity Demand
With an increasing number of users, a plurality of User Equipments (UEs) need to be scheduled concurrently, particularly in Multi-User Multiple Input Multiple Output (MU-MIMO) and Coordinated Multi-Point Transmission/Reception (CoMP). In the course of scheduling Rel-10 or higher-release UE, Downlink Control Information (DCI) format 2C or downlink control information format at a higher overhead is largely used.
Considering of controlling CRS overhead, a system subsequent to the Rel-10 is largely configured with a Multimedia Broadcast Single Frequency Network (MBSFN) sub-frame in which there are at most two symbols that can be used for transmission of PDCCHs, thus making the capacity of PDCCHs further insufficient.
In Carrier Aggregation (CA), cross-carrier scheduling may be performed by scheduling transmission of a plurality of Component Carriers (CCs) over a PDCCH of one CC, thus making control resources further insufficient.
2) Interference Suppression Problem
With PDCCH and PDSCH multiplexed through TDM, it may be difficult to obviate inter-cell PDCCH interference by allocating resources.
In a Heterogeneous Network (HetNet) scenario, Home eNodeB (HeNB) and other nodes are usually deployed without reasonable network design, and interference between PDCCHs may become more serious.
3) Coverage Extension Demand
Along with expansion of LTE network deployment, the coverage of the LTE system will be extended gradually from the urban area to the suburb and the exurb and even to the countryside, so the edge coverage capability of the control channel needs to be taken into account; and in the CoMP, the UE tends to be located at the boundary of a cell, and a better edge coverage capability of the control channel is also desired.
For the UE at the edge of a cell, PDCCHs tend to be aggregated at a her level, thus the capacity of PDCCHs will be more limited.
4) Influence Due to an Increasing Number of Antennas
Only 4 antenna ports at most can be supported in LTE Rel-8/9, and at most 8 antenna ports can be supported in LTE Rel-10 specification. Along with expansion of LTE network deployment, the number of antennas at the system side will also be upgraded gradually from single-antenna configuration mode to 2, 4 or 8-antenna configuration mode. With an increasing number of antennas, a more flexible multi-antenna transmission method can be used with PDSCH, and PDCCH can only be transmitted based upon a single port or transmit diversity of 1/2/4 CRS port. Considering of controlling CRS overhead, it is very likely for the Rel-10 and later releases to configure only two CRS ports. In this case, transmission performance of the PDCCH may be further insufficient.
With an increasing number of antennas, a CRS needs to be mapped to a real antenna port through antenna virtualization. In order to ensure a sector direction to be covered effectively by sector shaping as a result of weighted integration of a plurality of array elements, some constraint needs to be put on the design of the array elements and the selection of a weighting vector. In this case, the sector shaping as a result of virtualization may have an adverse influence on the coverage at the edge.
As can be apparent, the existing PDCCH can only be transmitted at a single CRS-based layer, thus the efficiency of transmission is limited; it is very likely for the Rel-10 and later releases to configure only two CRS ports, and in this case, PDCCH can only be transmitted through SFBC, thus the efficiency of transmission will be more limited; and the process of mapping a CRS port to a physical antenna through antenna virtualization may have an adverse influence upon the coverage at the edge of the PDCCHs.